Conventionally, a controlled current producing differential circuit apparatus has been used for obtaining a controlled current.
Referring now to FIG. 1, an example of the conventional controlled current producing differential circuit apparatus will be described. An equivalent of the conventional controlled current producing differential circuit apparatus is, for example, disclosed in Japanese Patent No. P56-42169 B2 (Tokkaisho 56-42169) issued on October 2, 1981. In FIG. 1, the conventional controlled current producing differential circuit apparatus is provided with a differential circuit 10, a control voltage supply circuit 12, a reference voltage supply circuit 14 and a power voltage supply source 16. The control voltage supply circuit 12 and the reference voltage supply circuit 14 supply the differential circuit 10 with a control voltage Vc and a prescribed reference voltage Vr, which are described later. The power voltage supply source 16 has a prescribed power source voltage Vcc.
The differential circuit 10 is constituted by a pair of transistors 18a and 18b and a current source 20. The current source 20 supplies an input current Iin subject to control by the control apparatus. The emitters of the transistors 18a and 18b are connected to the current source 20 through emitter resistors 22a and 22b, respectively. The bases of the transistors 18a and 18b are coupled to the control voltage supply circuit 12 and the reference voltage supply circuit 14 through resistors 24a and 24b, respectively. Further, the base of the transistor 18a is coupled to the reference voltage supply circuit 14 through a resistor 24c.
The control voltage supply circuit 12 is constituted by a variable resistor 26 coupled across the power voltage supply source 16. An output terminal 26a of the variable resistor 26 is coupled to the differential circuit 10 through an output terminal 12a for outputting the control voltage Vc.
In control voltage supply circuit 12, the output terminal 12a outputs the control voltage Vc. When assuming the impedance Z26 of the variable resistor 26 is sufficiently lower than the resistance R24a of the resistor 24a (Z26&lt;R24a), the control voltage Vc is given as follows in reference to the power source voltage Vcc of power voltage supply source 16 and the control ratio .alpha. of the variable resistor 26; EQU Vc=.alpha..Vcc (1)
where .alpha. is a fraction less than one (0.ltoreq..alpha..ltoreq.1).
The reference voltage supply circuit 14 is constituted by a transistor 28, two diodes 30a and 30b and three resistors 32a, 32b and 32c. The collector of the transistor 28 is coupled to the positive terminal of the power voltage supply source 16. The emitter of the transistor 28 is coupled to the negative terminal of the power voltage supply source 16 through the resistor 32a. The base of the transistor 28 is coupled to the positive terminal of the power voltage supply source 16 through the resistor 32b. Further, the base of the transistor 28 is coupled to the negative terminal of the power voltage supply source 16 through a series circuit of the diodes 30a, 30b and the resistor 32c. The emitter of the transistor 28 is coupled to the differential circuit 10 through a reference voltage output terminal 14a for outputting the reference voltage Vr.
In an integrated circuit configuration, the diodes 30a and 30b are made by a form of transistor, similar to the transistor 28 and the like. Therefore, the forward voltage drop of diodes, such as the diodes 30a and 30b, and the forward base-emitter junction voltage of transistors, such as the transistor 28, present the same prescribed voltage Vf. Therefore, the emitter potential of the transistor 28 becomes equal to the potential on the connection node between the diodes 30a and 30b. Provided the resistances R32b and R32c of the resistor 32b and 32c are equal to each other (R32b=R32c), the potential on the connection node becomes half of the power source voltage Vcc of power voltage supply source 16. As a result, the reference voltage Vr obtained on the emitter of the transistor 28 is given as follows: EQU Vr=(1/2).Vcc (2)
The control voltage Vc and the reference voltage Vr are applied to the bases of the transistors 18a and 18b through the resistors 24a and 24b, respectively. When assuming the impedance Z26 of the variable resistor 26 is sufficiently lower than the sum of the resistances R24a and R24c of the resistors 24a and 24c (Z26&lt;(R24a+R24c)), the base potential Vba of the transistor 18a is given as follows; EQU Vba=[R24c/(.alpha..R24a+R24c)].(Vc-Vr)+Vr
The base potential Vbb of the transistor 18b is also given as follows; EQU Vbb=Vr (4)
When assuming the resistances R24a and R24b have the same resistance R24, the potential difference .DELTA.V between the base potentials Vba and Vbb is given as follows; ##EQU1##
The potential difference .DELTA.V, therefore, varies in accordance with the control ratio .alpha. of the variable resistor 26. This is done by controlling the variable resistor 26. When assuming the potential difference .DELTA.V is sufficiently smaller than the voltage drops V22a and V22b across the emitter resistors 22a and 22b, respectively, collector currents Ic.18a and Ic.18b of the transistors 18a and 18b vary differentially in proportion to the potential difference .DELTA.V.
As a result, the conventional differential circuit control apparatus can control the output currents Ic.18a and Ic.18b in accordance with the control of the variable resistor 26.
However, the conventional controlled current producing differential circuit apparatus has a drawback as follows. As shown by Equation 5, the potential difference .DELTA.V is not only a function of the control ratio .alpha., but also a function of the power source voltage Vcc. Therefore, the output current obtained as the collector current flowing through the transistor 18a and/or 18b varies unexpectedly, if the power source voltage Vcc is not stabilized.
Further, the resistances R22a and R22b of the emitter resistors 22a and 22b are required to be relatively large for causing voltage drops V22a and V22b sufficiently larger than the potential difference .DELTA.V. However, the varying range of the output current I18a and/or I18b is reduced in reverse proportion to the resistances R22a and R22b of the emitter resistors 22a and 22b.
If the resistances R22a and R22b of the emitter resistors 22a and 22b are decreased for enlarging the varying range of the output current, the potential difference .DELTA.V is apt to be affected by the thermal voltage Vt of transistors, such as the transistors 18a and 18b. The thermal voltage Vt of transistors is given as follows; EQU Vt=K.T/q
where K is Boltzman's constant, T is absolute temperature and q is the electric charge of an electron.
Therefore, the output current I18a and/or I18b varies due to not only changes in the power supply voltage Vcc but also changes in temperature. Further, the varying ranges of the output current I18a and/or I18b are affected by the impedance of the control voltage supply source 12 and the reference voltage supply source 14.